The present invention relates to very precise and stable metal foil resistor elements intended for R-standards, and also to high precision devices, such as multi-meters, R-bridges, R-decade boxes, R-potentiometers and other resistor networks, which may use such resistor elements and which thereby would obviate the need for frequent control tests and retrimming by using supplemental R-standard resistors.
A wide variety of applications could advantageously use such precise and stable resistor elements. The existing resistors, having precise and low temperature coefficients of resistance (TCR), are very small and are therefore easily incorporated into electronic instruments; however, they cannot be used as R-standards, e.g., as SR104 of ESI R-standard, because of their insufficient accuracy and stability (shelf life, power and thermal shocks), and because of their low rated power dissipation. On the other hand, the known R-standard resistors having high precision, high stability and high power dissipation, are very expensive, are of large size, are time consuming for steady state conditions, and generally are intended only for checking and measuring electrical circuits under laboratory conditions.
The absence of such resistor elements of high precision, high stability, relatively high power dissipation, and low cost impedes the development and design of many instruments, which in turn impedes the development of measuring techniques.
The majority of the shortcomings in the existing photo-etched metal foil resistors are caused by the following:
a. the small size of the chips, and accordingly the small width of the lines and spaces of the resistive patterns path: thus, the deleterious effects induced by the "notches", "holes" and "bridges" upon R-stability, resulting from the exiting metal-foil and photo-etching technology, are increased when decreasing the width of the lines and the spaces, depending on the thickness and the quality of the foil and the standard of photo-etching procedure; also, the small surface of the chip does not allow to dissipate high power; PA1 b. the influences of the surrounding environment: for example, oil absorbed at elevated temperatures by cement layers, and their shrinkage and "creep" occurring at high shear stresses, cause substrate and metal foil pattern deformations and resistance deviation. PA1 a. Pattern--a metal foil resistive element, photo-etched or etched in an other way, bonded to an insulating substrate and comprising series connected non-trimming (i.e., nonadjustable resistive part, trimming steps or fired and termination pads (e.g., conductive platings) for external connections. PA1 b. TCR-characteristic--the temperature dependence of relative resistance change of the bonded metal foil pattern(s) of resistance R, i.e. ##EQU1## T.sub.o --is the middle of the given temperature range of resistor elements; .DELTA.T.sub.f, .DELTA.T.sub.s --are the temperature deviations of the pattern foil and substrate from the middle temperature T.sub.o ; and R=R(T.sub.o, .DELTA.T.sub.f, .DELTA.T.sub.s)-R(T.sub.o) --are the resistance changes due to temperature changes .DELTA.T.sub.f and .DELTA.T.sub.s of the pattern foil and substrate, to which it is bonded. PA1 c. Slope S(T.sub.o)--is the slope of the TCR-characteristic at the middle temperature T.sub.o : ##EQU2## d. Curvature C(T.sub.o) is the curvature or nonlinearity of the TCR-characteristic at the middle temperature T.sub.o : ##EQU3## for precision resistors. e. R(T.sub.o)=.rho.(T.sub.o).times.l.sub.o is the Ohm's law for a bonded to insulating substrate unstrained metal foil strip (pattern) at initial temperature To, where .rho.(T.sub.o)--is the resistivity of the metal foil material at initial temperature T.sub.o equals the resistance per one foil's square, ohms/square; l(T.sub.o) --is the initial length of a strip (path of pattern) in units of squares. PA1 -K(.alpha..sub.f .DELTA.T.sub.f -.alpha..sub.s .DELTA.T.sub.s)--is the relative resistance change of the thermostressed bonded metal foil due to its strain- effect, .alpha..sub.f, .alpha..sub.s --are coefficients of thermal expansion of materials of metal foil and substrate; and PA1 K - is the strain gauge factor of the foil for uniform temperature deformation, dependent upon the foil material and is usually close to 2. PA1 f. Two sandwiched patterns of the same selfheating. PA1 (7) Basic pitch: p=l+s, and squares/pitch=p/(p-s), where l and s are the widths of the line and space of the path, and determine the overwhelming component of pattern resistance for the given resistivity of film and pattern surface. In this and in other cases of the same selfheatings, the following correlations can be written: ##EQU7## where: lower indexes 1 and 2 concern the first and the second patterns; PA1 (11) R(T.sub.o)=R.sub.1 (T.sub.o)+R.sub.2 (T.sub.o) - sum of the resistance of two patterns at temperature T.sub.o ; ##EQU8## g. R-trimming step--a discrete part of the R-trimming section of a metal foil resistive pattern which is series connected to its non-trimming parts. It includes a resistive pattern of the step, and its shunt that can be cut and recovered (by soldering or welding), if the shunt is copper-nickel-gold plated and has sufficient size; in these ways, the pattern's resistance R is increased or decreased by the given step value. PA1 h. TCR-trimming step--a part of a copper-nickel-gold plated TCR trimming section of the metal foil resistive pattern is series connected to its non-trimming parts, and includes a temperature sensitive step pattern of relative low resistance, and a shunt that can be cut and recovered (by soldering or welding, if it has sufficient size); in these ways the slope S of the pattern's TCR-characteristic is increased or decreased by given step value. PA1 i. Copper-nickel-gold plating--the plating of the given region, and formed of a material selected from the group consisting of copper, nickel and gold. PA1 j. Foil batches--metal foils produced from the same alloy batch but subjected to different thermal treatments in order to obtain different slopes of TCR-characteristics by keeping the curvature and resistivity substantially constant. PA1 k. Foils modification--foils produced from the same basic alloy but with special alloying element(s) in order to change the sign and curvature value of the basic TCR-characteristic.
An object of the present invention is to provide a resistor element having important advantages in the above respects.